The present invention relates to an analog-digital coder comprising a charge transfer coded voltage generator.
It is well known in the art to determine by successive approximations the coefficients a.sub.o, a.sub.1 . . . a.sub.i . . . a.sub.n, equal to 0 or 1, making it possible to digitally code an unknown analog voltage V.sub.x by writing it in the form: EQU .vertline.V.sub.x .vertline.=a.sub.o .multidot.V.sub.R +a.sub.1 .multidot.V.sub.R /2+a.sub.2 .multidot.V.sub.R /2.sup.2 + . . . +a.sub.i .multidot.V.sub.R /2.sup.i + . . . +a.sub.n .multidot.V.sub.R /2.sup.n
in which V.sub.R is a reference voltage. For this purpose:
.vertline.V.sub.x is firstly compared with V.sub.R --if .vertline. Vx is less than V.sub.R, then a.sub.o is equal to 0, otherwise it is equal to 1;
.vertline.V.sub.x .vertline. is then compared with V.sub.R1 =a.sub.o V.sub.R +V.sub.R /2--if .vertline.VV.sub.x .vertline. is less than V.sub.R1, then a.sub.1 is equal to 0, otherwise a.sub.1 is equal to 1;
then V.sub.x .vertline. is compared to V.sub.R2 =a.sub.o V.sub.R +a.sub.1 .multidot.V.sub.R /2+V.sub.R /4.multidot.--if .vertline.V.sub.x .vertline. is below V.sub.R2, then a.sub.2 equals 0, otherwise a.sub.2 equals 1;
and so on until all the coefficients a.sub.o . . . a.sub.n have been determined.
Thus, for determining by successive approximations the coefficients a.sub.o . . . a.sub.n it is necessary to have the voltages V.sub.R and EQU V.sub.R1 =a.sub.o V.sub.R +a.sub.1 .multidot.V.sub.R /2+a.sub.2 .multidot.V.sub.R /2.sup.2 + . . . +a.sub.i-1 .multidot.V.sub.R /2.sup.i-1 +V.sub.R /2.sup.i with i=1 . . . n.
We know a charge transfer coded voltage generator which is shown in FIG. 1. It will be described in greater detail hereinafter and supplies the voltages V.sub.R and V.sub.Ri. This generator for example is described in U.S. Pat. No. 4,350,976, assigned to Thomson-CSF.
A reference charge quantity 2Q.sub.R is injected into the generator at the start of the processing of each sample .vertline.V.sub.x .vertline.. This generator is constituted by a charge transfer device (CTD), which is divided up into two parallel channels. Half the charge in the CTD before division is collected in each channel.
A charge reading device (CRD) is connected to the two channels--to the storage grid G.sub.1 following the division for the first channel and to the third storage grid G.sub.4 following the division for the second channel.
The CRD then collects the quantity of charges Q.sub.R beneath G.sub.1 and supplies the voltage V.sub.R and then coefficient a.sub.0 can be determined. If a.sub.0 =0 the quantity of charges Q.sub.R stored in the second channel is discharged and if a.sub.0 =1 this charge quantity is stored beneath the second storage grid G.sub.3 following the division of the CTD.
In all cases the charge quantity Q.sub.R stored beneath grid G.sub.1 of the first channel performs a round trip on either side of the zone where the CTD is divided into two channels and thus a charge quantity Q.sub.R /2 is stored beneath grids G.sub.1 and G.sub.2.
Thus, after the transfer of the possible content of G.sub.3, which will be called a.sub.0 Q.sub.R to beneath G.sub.4, the CRD collects a charge quantity equal to a.sub.0 Q.sub.R +Q.sub.R /2 and thus supplies V.sub.R1 making it possible to determine a.sub.1. This is continued until the determination of a.sub.n takes place after the processing of V.sub.Rn.
The following problems occur when it is desired to provide an analog-digital coder with the charge transfer coded voltage generator in question. The coded voltage generator has the defects inherent in charge transfer devices and which essentially consist of the inefficiency of transfer, the dark current and the leakage current resulting from crystal imperfections or interface impurities (and particularly occurring with surface transfer CTD). However, the scale of these defects increases in the case of a coded voltage generator where an initial charge quantity 2Q.sub.R has to perform n round trips on either side of the division of the CTD to finally obtain Q.sub.R /2.sup.n and where the storage time for this charge quantity is therefore relatively long.
With regard to the inefficiency of transfer it is possible to use a CTD with a buried channel, whose transfer coefficient is substantially equal to 1, to within a few 10.sup.-5 units.
However, with regard to the dark and leakage currents it is necessary to limit the number n of successive divisions which can be validly performed, whilst retaining a final charge which is differentiated in a completely satisfactory manner from the thermal background noise.
A further problem is connected with the offset voltages appearing at all the active members adjacent to the generator for forming the coder, particularly when these members are constructed according to MOS technology.